(Father Robert Brennan with his late accuser, Sean McIlmail, in family photo. Photo provided.)

By Mark Abrams

PHILADELPHIA (CBS) — The family of a Willow Grove, Pa. man who died of an accidental drug overdose a month ago (see related story) — after cooperating with authorities in bringing charges in a clergy sex abuse case — has filed a lawsuit against the Archdiocese of Philadelphia and the young man’s alleged abuser.

Attorneys for the family of 26-year-old Sean McIlmail insist it’s not about the money, but the search for truth and justice for all victims of clergy sex abuse in the archdiocese.

Sean’s mother, Deborah, says the family is still trying to cope with the loss.

“Sean appeared good on the outside,” she said today, “but on the inside, he didn’t believe in himself. The unspeakable, disgusting horrors that had happened to Sean by Father Robert L. Brennan would haunt Sean forever.”

Deborah McIlmail alleges that the abuse by Brennan lasted four years and began in 1993, when Sean was 11 and attending Resurrection of Our Lord Parish school in Rhawnhurst.

Days after McIlmail’s death last month, Philadelphia DA Seth Williams reluctantly dropped the case against Brennan.

Brennan was labeled a serial sexual predator and mentioned in two grand jury reports about clergy sexual abuse in the archdiocese.

The suit also names Msgr. William Lynn, former secretary of clergy in the archdiocese, for failing to act to protect McIlmail.

Lynn is currently serving to three to six years in state prison for his conviction for covering up clergy sex abuse while serving in his management role of priests from 1992 to 2004.